Loans and Allowance for Loan Losses - Loan Summary (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 1,374,257,000	$ 1,323,258,000
Less: Allowance for loan losses	(6,477,000)	(5,860,000)	$ (5,105,000)
Loans, net	1,367,779,622	1,317,397,909
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	29,836,000	32,794,000
Less: Allowance for loan losses	0	(16,000)
Loans, net	29,836,000	32,778,000
All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,344,421,000	1,290,464,000
Less: Allowance for loan losses	(6,477,000)	(5,844,000)
Loans, net	1,337,944,000	1,284,620,000
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	663,694,000	642,988,000
Less: Allowance for loan losses	(2,925,000)	(2,465,000)	(2,369,000)
Commercial Real Estate [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	16,236,000	17,903,000
Less: Allowance for loan losses		(16,000)
Commercial Real Estate [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	647,458,000	625,085,000
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	299,147,000	293,457,000
Less: Allowance for loan losses	(1,519,000)	(1,596,000)	(1,382,000)
Consumer Real Estate [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	6,985,000	7,450,000
Less: Allowance for loan losses		0
Consumer Real Estate [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	292,162,000	286,007,000
Construction and Land Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	142,704,000	135,409,000
Less: Allowance for loan losses	(627,000)	(521,000)	(717,000)
Construction and Land Development [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	5,003,000	5,120,000
Less: Allowance for loan losses		0
Construction and Land Development [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	137,701,000	130,289,000
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	256,333,000	238,087,000
Less: Allowance for loan losses	(1,210,000)	(1,062,000)	(520,000)
Commercial and Industrial [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	649,000	858,000
Less: Allowance for loan losses		0
Commercial and Industrial [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	255,684,000	237,229,000
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	12,379,000	13,317,000
Less: Allowance for loan losses	(196,000)	(216,000)	$ (117,000)
Consumer and Other [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	963,000	1,463,000
Less: Allowance for loan losses		0
Consumer and Other [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 11,416,000	$ 11,854,000